Mixamo/Animation Merge animations
You need to put all mixamo animations to a single armature using nla tracks.
I made a script for automating it.
You can try a script that i wrote. It combines all .fbx anim files that you downloaded from mixamo into one armature.
gumroad.com/l/MixAnimMerger
You can then skin the mesh to the new armature and export it as fbx.it will contain all animations.